VI (Visual) is a display oriented interactive text editor. When using VI the screen of the terminal acts as a window into the file which is being editing. Changes which made to the file are reflected in what is seen. Using VI the user can insert new text any place in the file quite easily. Most of the commands to VI move the cursor around in the file. There are commands to move the cursor forward and backward in units of characters, words, sentences and paragraphs. A small set of operators, like d for delete and c for change, are combined with the motion commands to form operations such as delete word or change paragraph, in a simple and natural way. This regularity and the mnemonic assignment of commands to keys makes the editor command set easy to remember and to use. VI works on a large number of display terminals, and new terminals are easily driven after editing a terminal description file. While it is advantageous to have an intelligent terminal which can locally insert and delete lines and characters from the display, the editor will function quite well on dumb terminals over slow phone lines. The editor makes allowances for the low bandwidth in these situations and uses smaller window sizes and different display updating algorithms to make best use of the limited speed available. It is also possible to use the command set of VI on hardcopy terminals, storage tubes and 'glass ty's' using a one line editing window; thus VI's command set is available on all terminals. The full command set of the more traditional, line oriented editor ED is available within VI; it is quite simple to switch between the two modes of editing